Paints are applied to many objects and serve mostly the purpose of enhancing protection and aesthetics. Paint can be applied to a surface in many different ways which, to a certain extent, depends on the substrate and functionalities it will have. Examples are chemical and physical vapor deposition, dip-coating, spraying, and roll-to-roll coating.
When the paint is applied, the paint material is initially a liquid layer with (solid) paint particles immersed therein. This allows bringing the paint particles to the surface which is to be coated (e.g. by spraying in the case of spray painting). Once on the surface, the liquid does not have a further purpose. Nevertheless, it is still part of the paint layer which makes that painted surfaces have to dry (e.g. by their wet parts evaporating or by curing). This requires some waiting time, which in industrial processes is economically unfavorable. While increasing the temperature can somewhat accelerate this process, the available temperatures are generally limited, since at elevated temperatures paint cures differently which can involve undesired changes in e.g. the final visual appearance.
Nowadays, large production plants in which paint layers are applied to future products typically comprise paint process lines at which paint robots spray the paint. Although the painting process thus seems to be largely automatized, still many painted objects show failures in the thickness or visual appearance. For these reasons, accurate quality control of paint layers is an important part of the paint process.
For this purpose, prior art techniques such as acoustic and magnetic sensing have been developed for determining the thickness of paint layers. However, these techniques only work in contact mode, which is generally undesired and not always applicable without risk of damaging the paint layer. In addition, recently methods based on THz radiation have been proposed. For example, JP 2004028618 A and EP 2213977 A1 describe respective methods for determining the thickness of a dry paint film using THz radiation. The thickness is obtained by subtraction of peak positions of a time-domain signal. The peak positions, together with a known group index of refraction of the wet paint, allow calculation of the thickness. However, the robustness of this method leaves room for improvement.
In addition, the above-mentioned methods only address the problem of determining the thickness of a paint layer after the paint has dried.